Retrofit Airtightness: How to Seal an Existing House Without Ripping It Apart

You can dramatically improve the airtightness of an existing house without stripping it back to brick. The trick is knowing where old homes actually leak, working room by room in a sensible order, and using an internal liquid applied membrane so the airtight layer goes over what is already there — existing plaster included — instead of requiring walls to come down. This guide covers the why, the where and the how, plus the one caution that matters most in retrofit: moisture.

Why old homes leak

A typical pre-1990s UK house was never designed with an air barrier. Air moves through bare masonry and old lime or gypsum plaster cracks, up chimneys and disused flues, around suspended timber ground floors, through the band of joist ends built into masonry at each floor, past window and door reveals, and through decades of accumulated holes: boiler flues, waste pipes, cables, extractor fans, loft hatches. Dot-and-dab plasterboard, where fitted, adds a connected air void behind every board. None of these leaks is individually dramatic, but together they can add up to the equivalent of leaving a window open all winter — heat you pay for, leaving through the fabric.

First, a plan: follow the air barrier line

Before sealing anything, decide where your airtight line runs — for most retrofits, it is the internal face of the external walls, the ground floor, and the top-floor ceiling. Draw it on a simple section of the house. Everywhere the line breaks (stairwells, floor voids, chimney breasts, bay windows) is a detail to solve. If you would rather have that done for you, our free drawing markup service will mark the airtight line and the risky junctions on your own drawings before you start.

The room-by-room approach

• Loft and top-floor ceilings. Usually the biggest wins per hour of work. Seal the loft hatch with compression seals, seal around soil vent pipes, cables and downlight penetrations, and make the ceiling plane continuous before topping up insulation.
• Ground floor. Suspended timber floors leak at every board gap and around the skirting perimeter. Sealing the floor-to-wall junction with airtight sealant and treating accessible floor perimeters makes a measurable difference without lifting the whole floor.
• Windows and doors. Seal frame-to-wall junctions at every reveal — airtight tapes or a brush-applied membrane bead close the gap that foam alone never seals properly.
• Chimneys and flues. Disused flues are motorways for warm air. Cap ventilated (a small vent prevents damp in the stack) or fit a removable chimney balloon where the fireplace stays.
• Services. Work through every penetration: boiler flue, waste pipes, meter entries, extractor fans, cable runs. Seal each with sealant or tape while you are in the room.

The internal liquid membrane: sealing walls without removing them

The step that transforms a leaky house rather than nibbling at it is making whole wall and ceiling planes airtight in one go. This is where liquid applied membranes earn their place in retrofit: they are applied directly onto the existing substrate — old plaster, blockwork, brick — with no need to remove walls, batten out, or fix sheet membranes to anything.

Passive Purple is applied as a two-coat system by spray or brush, forming one seamless airtight and vapour control layer across the wall, into reveals and across junctions. It is Passivhaus certified and BBA approved, with tested air leakage of 0.02 L/s·m² at 75Pa (ASTM E2178) — retrofit does not have to mean second-rate airtightness. Sprayed, one installer can cover up to 300m² a day, so a whole house interior is realistic within a short possession of each room — decorate over it afterwards and the airtight layer disappears into the build-up. Preparation still matters: surfaces should be sound and dust-free, and friable or painted substrates may need one of our primers first. For patching, reveals and single-room jobs without spray kit, the brush grade in the Passive Purple range covers the detailing, and Airtight White offers a white finish where the treated surface will remain visible.

How far to go: the EnerPHit context

For deep retrofits, the Passivhaus Institut's EnerPHit standard targets 1.0 ACH @50Pa — close to the 0.6 ACH new-build Passivhaus target and far tighter than a typical existing house. You do not need to certify to benefit: every step towards those numbers cuts draughts and bills. But EnerPHit shows what is achievable in existing buildings when the air barrier is continuous, which in practice means treating airtightness as a whole-house layer, not a list of draught-proofing jobs.

The condensation caution: vapour matters as much as air

Tighten an old house and you change how it handles moisture — this is the part retrofit gets wrong most often. Two rules keep you safe. First, ventilate deliberately: as uncontrolled leakage falls, upgraded extract fans or MVHR must take over supplying fresh air, or humidity will rise and condensation follows. Second, respect the wall's need to dry. Solid walls in particular must not be sealed with vapour-closed layers in the wrong place; the airtight layer on the warm side should control vapour without trapping moisture in the fabric. This is exactly why an intelligent airtight vapour control layer belongs on the internal face — it stops warm, moist indoor air being pumped into the cold wall, which is the main condensation risk in a tightened house. If your project involves internal wall insulation or solid walls, get the moisture design checked before sealing.

Frequently asked questions

Can I make an old house airtight without removing plaster?

Yes. A liquid applied membrane goes directly over sound existing plaster or masonry as a two-coat system, forming a continuous airtight layer you then decorate over. Loose, friable or heavily painted surfaces need preparation or a primer first.

What order should I tackle a retrofit airtightness job in?

Plan the airtight line first, then work: loft and top ceilings, ground floor junctions, window and door reveals, chimneys, service penetrations — and finally (or alongside) the wall planes with a liquid membrane. Test or smoke-check as you go if you can.

Will making my house airtight cause damp?

Not if you do the other half of the job. Airtightness reduces moisture being carried into the fabric by leaking air, but the house must have designed ventilation to remove indoor moisture, and the airtight layer should provide vapour control on the warm side. Skipping the ventilation upgrade is what causes problems.

Do I need to hit a specific airtightness number in a retrofit?

Building Regulations airtightness limits apply to new dwellings; for most retrofits there is no mandatory test figure unless you are certifying (EnerPHit targets 1.0 ACH @50Pa). Treat every improvement as a win — the goal is fewer draughts, lower bills and a fabric that stays dry.